1. Field of the Invention
The present invention relates to electromagnetic wave sources utilizing rectangular sheet electron beams or annular electron beams, and more particularly, to an electron beam collector for such devices which allows for recovery of spent beam energy by the application of device geometry and voltage depression.
2. Background Art
Linear beam electron devices are used in molecular spectroscopy, surveillance devices and sophisticated communications and radar systems that require generation or amplification of radio frequency or microwave electromagnetic signals. In UHF transmitters, the klystron is the most common power output device. However, a klystron with no efficiency optimizing circuitry is typically only 40 to 50% efficient depending on the type of device used. One scheme devised to improve klystron efficiency is the depressed collector, which allows for the recovery of energy from the electron stream rather than dissipating the energy as heat.
Depressed collectors have routinely been incorporated in cylindrically symmetric vacuum tubes, such as traveling wave tubes and klystrons for more than 40 years. Depressed collectors for cylindrical devices are produced by many companies including Communications and Power Industries, Northrop-Grumman, Hughes and Teledyne.
Previous implementations of depressed collectors have utilized the space charge forces in the electron beam to expand the electrons radially where they can be collected on surfaces that are facing in a direction away from the main body of the device. Other implementations use variations of the magnetic field to prevent return of secondary electrons.
Despite the advantages of depressed collectors, no implementations of such devices have been reported for use with sheet electron beams, such as those which are utilized in submillimeter frequency Backward Wave Oscillators. The current generation of submillimeter frequency Backward Wave Oscillators typically operate with efficiencies of less than five percent. Consequently, most of the energy in the electron beam is deposited in the collector of the device where it must be dissipated as thermal power and typically requires cooling. The successful implementation of a depressed collector would allow for recovery of most of the electron beam energy, reducing the electrical power requirements for the device and reducing the thermal power that must be dissipated.
Unfortunately, many devices including the submillimeter frequency Backward Wave Oscillator are not amenable to the techniques utilized to implement depressed collectors in the prior art such as cylindrically symmetric vacuum tubes, traveling wave tubes and klystrons. Simple depression of the existing collector geometry results in excessive generation of backscattered electrons and true secondary electrons. At the voltages used in the submillimeter frequency Backward Wave Oscillator device, the secondary electron yield can approach one, meaning that almost as many secondary electrons are generated as there are incoming primary electrons. The secondary electrons are accelerated at full voltage and impact on the main body of the device severely limiting the amount of energy that can be recovered from the electron beam.
An additional complication is the rectangular geometry of a device utilizing a rectangular sheet electron beam. The rectangular geometry is not amenable to the cylindrical geometrical configurations utilized in the prior art.
Finally, the Backward Wave Oscillator device operates at small currents (typically 30 to 40 milli-amperes) where space charge forces are small and techniques to control beam spread are limited by the presence of a strong, uniform magnetic field. Nor is the device amenable to the use of induced variations in the magnetic field to achieve beam spread because the distance between the body of the device and the collector is inadequate in that the distance is less than 100 microns. Variations of the magnetic field on this scale would be difficult to implement without adversely affecting beam transmission through the circuit.